1. Field of the Invention
The present invention relates to a liquid crystal display.
More particularly, the present invention relates to a liquid crystal display using a display mode which controls scattering of light generated at an interface between liquid crystals by changing a difference in refractive index between different liquid crystals by an external voltage.
2. Description of the Related Art
Conventionally, there has been practically used a twisted nematic (TN) type or a super twisted nematic (STN) type display using a nematic liquid crystal as a display element, in which an electro-optical effect is used. In addition, it has been proposed to use a ferroelectric liquid crystal. These need a polarizing plate and orientation process. Meanwhile, there are a dynamic scattering (DS) effect and a phase change (PC) effect which use scattering of a liquid crystal but do not need the polarizing plate. Recently, there has been proposed a method which electrically controls transparent or white-opaque state using birefringent properties without the polarizing plate and the orientation process. This method is a display mode in which basically, when a voltage is applied, the liquid crystal is uniformly oriented in the direction of an electric field and the transparent state is displayed by matching ordinary refractive index of a liquid crystal molecule to a refractive index of a base polymer medium, and when the voltage is not applied, a light scattering state caused by disorder of the orientation of the liquid crystal molecule is used.
Although there is disclosed a method in National Publication of Translation of PCT Application No. 501631/1983, in which the liquid crystal is contained in a polymeric capsule, since the liquid crystal is an independent closed cell and it is surrounded by polymers, a space between electrodes is not effectively used so that a contrast is low. In addition, a driving voltage causing a change in the liquid crystal orientation is high and the range of utilization is small.
In addition, there is disclosed a method in National Publication of Translation of PCT Application No. 502128/1986, in which the liquid crystal is mixed with a photo-polymerization or a thermoplastic resin and the resin is hardened to precipitate the liquid crystal and then, liquid crystal droplets are formed in the polymer matrix. However, since the above method employs phase separation process, while a method for controlling a diameter of the liquid crystal droplet is disclosed in Japanese Unexamined Patent No. 72317/1991, it is difficult to precisely control the diameter of the liquid crystal droplet.
In addition, there is disclosed a method in Japanese Unexamined Patent No. 59515/1991, in which the liquid crystal is immersed with a polymeric porous film. Since the above method does not use the phase separation process when the liquid crystal droplets are formed, the degree of freedom of selection of applicable resin and liquid crystal is considerably large and the polymeric porous film can be widely employed. However, the liquid crystal droplet can not be sufficiently controlled under the present circumstances.
In addition, there is disclosed a method in Japanese Unexamined Patent No. 46621/1991, which is characterized in having a structure in which a polymer bead of a scattering source of light is dispersed between two transparent electrodes in the liquid crystal. Although intensity of light scattering is high, it is difficult to uniformly scatters the bead, so that the display is not likely to be uniform.
A liquid crystal display element according to the above prior art uses a fact that a difference between refractive indices of basically one kind of liquid crystal material and a polymeric material varies with a change in the external electric field.
It is defined that n.sub.e is a refractive index of extraordinary light and n.sub.o is a refractive index of ordinary light. In the liquid crystal display element using the conventional polymer dispersed mode, when the voltage is not applied, light is scattered using a difference between a refractive index n (=(n.sub.e +2n.sub.o)/3) in a random orientation state of the liquid crystal material and a refractive index n.sub.p of the polymeric material.
When the voltage is applied, the liquid crystal is orientated along the electric field, and then the refractive index of the liquid crystal changes to n.sub.o. At that time, when the refractive indices of the liquid crystal and the polymer are selected so as to realize n.sub.o =n.sub.p, the liquid crystal cell becomes transparent and then light is not scattered. Thus, the liquid crystal element controls the scattering state of light using a mismatch of the refractive indices of the liquid crystal and the polymer. However, even when the configuration of the liquid crystal droplet is optimized, if the mismatch of the refractive indices of the liquid crystal and the polymer is not sufficiently large, preferable contrast can not be obtained. Meanwhile, it is determined that the degree of the mismatch of the refractive indices of the liquid crystal and the polymer is in proportion to refractive index anisotropy .DELTA.n (=n.sub.e -n.sub.o) of the liquid crystal material. However, the refractive index anisotropy .DELTA.n of the actual liquid crystal material is 0.3 at most, which is not sufficient to obtain preferable contrast.
In addition, in order to make a polymer matrix, a volume ratio of a polymer to a liquid crystal component has to be 10% or more. Therefore, utilization of the liquid crystal volume in the cell is relatively low and it is difficult to improve the contrast. At the same time, the (effective) voltage becomes high with increasing the content of polymer, so that the voltage applied to the liquid crystal is lowered. As a result, the liquid crystal is confined in a small space, so that the driving voltage has to be highly set.
Although the contrast of a multi-layer cell formed of two layers can be high, it is necessary to provide a substrate material between the first and second layers to separate them, causing parallax. Furthermore, the multilayer cell needs electrodes, circuits or the like double because the first and second layers are separately driven. As a result, costs and weight are increased.